1. Field of the Invention
The present invention relates to a surface plasmon sensor used for quantitative analysis based on induction of surface plasmon, and more specifically, relates to a surface plasmon sensor which is capable of analyzing a liquid sample while being soaked under the liquid sample.
2. Description of the Related Art
Collective vibration of free electrons in metal causes compression waves called plasma waves. A quantized state of the plasma waves induced on a metal surface is called surface plasmon.
Heretofore, a variety of surface plasmon sensors have been suggested for use in quantitative analysis in which quantity of a substance contained in a sample is estimated based on an excitation phenomenon of the surface plasmon induced by optical waves. One of the most popular surface plasmon sensors is the one employing a system named the Kretschmann configuration (e.g. Japanese Unexamined Patent Publication No. 6(1994)-167443).
The surface plasmon sensor employing the Kretschmann configuration is basically constituted of a dielectric block in, for example, a prism-like shape, a metal film which is superposed on one surface of the dielectric block and is in contact with the sample, a light source for generating a light beam, an optical system which enables the light beam to realize a variety of angles of incidence on an interface between the dielectric block and the metal film, each of the angles satisfying a condition for total reflection on the interface, including those angles satisfying a condition for surface plasmon resonance, and photodetection means which detects a state of the surface plasmon resonance by measuring intensity of the light beam after the light beam is totally reflected at the interface.
A variety of angles of incidence as described above may be obtained with a light beam of a comparatively small beam diameter which is allowed to vary its angle of incidence with respect to the interface, or with a light beam of a comparatively large beam diameter which can be regarded as a group of beam components having different angles of incidence with respect to the interface and which can be made to focus on the interface. In the former case, the light beam is reflected with a variety of angles of reflection as the angle of incidence is varied, and the reflected light beam may be detected with a small, photodetector which moves in a motion synchronized with the change of the angle of incidence or may be detected with an area sensor which extends in an angular direction in which the angle of reflection varies. In the latter case, the reflected light beam may be detected with an area sensor which extends in a certain direction to receive all the beam components reflected with different angles of reflection.
In the surface: plasmon sensor having the above constitution, evanescent waves with a certain electrical field distribution are induced in the sample kept in contact with the metal film when the light beam reaches the metal film with a specific angle of incidence xcex8SP which is equal to or larger than the angle for total reflection, and then the evanescent waves excite the surface plasmon on an interface between the metal film and the sample. When wave number matching occurs, i.e. when the size of a wave number vector of the evanescent waves is equal to a wave number of the surface plasmon, the intensity of the light beam after being totally reflected at the interface between the dielectric block and the metal film drops sharply because the evanescent waves and the surface plasmon come into a resonance state and energy carried by the light beam is consumed by the surface plasmon.
The resonance as described above occurs only when the incident beam is a beam of p-polarized light. Thus, it is required to adjust a setting of the light beam in advance so that the light beam reaches the metal film as the beam of the p-polarized light. The wave number of the surface plasmon can be calculated from the angle of incidence xcex8SP at which the attenuated total reflection (ATR) occurs, and then a dielectric constant of the sample can be derived using the following relationship,             K      SP        ⁡          (      ω      )        =            ω      c        ⁢                                                      ϵ              m                        ⁡                          (              ω              )                                ⁢                      ϵ            s                                                              ϵ              m                        ⁡                          (              ω              )                                +                      ϵ            s                              
in which KSP is the wave number of the surface plasmon, xcfx89 is an angular frequency of the surface plasmon, c is the light speed in a vacuum, ∈m is a dielectric constant of the metal, and ∈s is the dielectric constant of the sample.
Once the dielectric constant of the sample ∈s is derived, density of a specific substance within the sample can be estimated based on an appropriate calibration curve etc. In summary, the quantitative analysis of the specific substance contained in the sample can be performed by measuring the angle of incidence xcex8SP at which the intensity of the reflected light beam drops sharply.
Beside the surface plasmon sensor of the above-described constitution, a surface plasmon sensor of another type called a long range type surface plasmon sensor is also widely known, the surface plasmon sensor realizing the quantitative analysis with higher sensitivity by providing a dielectric film layer which has a refractive index different from that of the dielectric block between the metal film and the dielectric block.
The existing surface plasmon sensors as described above have sometimes been used for measuring alcohol density of spirits during their manufacturing process, quality of water in a river, etc. However, there has been a problem in such cases that the existing surface plasmon sensors require an effort-taking procedure of sampling a small amount of the sample to be analyzed, e.g. the water in the river, and dropping the sample into a sample cell (a recessed portion for holding the sample therein) to hold the sample therein so that the sample comes into contact with the metal film.
The object of the present invention is to solve the problem by providing a surface plasmon sensor with which a mass of a liquid sample may be analyzed with simple procedures.
A first surface plasmon sensor according to the present invention is constituted of a dielectric block, a metal film, a light source for generating a light beam, an optical system, and photodetection means the same as the existing surface plasmon sensor described above, but the dielectric block, the light source, the optical system and the photodetection means are housed in a box having a waterproof structure keeping the metal film on the dielectric block in contact with an external atmosphere.
A second surface plasmon sensor according to the present invention is constituted of a dielectric block, a metal film, a dielectric film layer provided between the metal film and the dielectric block, a light source for generating a light beam, an optical system, and photodetection means the same as the existing long range type surface plasmon sensor described above, but the dielectric block, the dielectric film layer, the light source, the optical system and the photodetection means are housed in a box of having waterproof structure keeping the metal film on the dielectric block in contact with an external atmosphere.
An appropriate metal film to be used in the surface plasmon sensor according to the present invention may be selected from a group :of metal films including a gold film, a silver film, a copper film and an aluminum film.
In the surface plasmon sensor of the present invention, it is desirable to form a replaceable sensor chip constituted of at least a part of the dielectric block and the metal film superposed thereon.
Further, in the surface plasmon sensor of the present invention, a bonding substance which bonds to a specific substance may be applied fixedly on a surface of the metal film. A pair of an antigen and an antibody, the pair which is capable of antigen-antibody reaction, is one example of a combination of the specific substance and the bonding substance.
With the first surface plasmon sensor according to the present invention, the liquid sample may be easily brought into contact with the metal film by simply soaking the box having the waterproof structure under the liquid sample, because the dielectric block, the light source, the optical system and the photodetection means are housed in the box keeping the metal film on the dielectric block in contact with the external atmosphere. In this state, the surface plasmon sensor is able to perform the analysis of the sample in the same way as the existing surface plasmon sensor does.
As the box in which the dielectric block, the light source, the optical system and the photodetection means are housed has a waterproof structure, the liquid sample does not penetrate into the inner area of the box to have any detrimental effect on the dielectric block, the light source, the optical system or the photodetection means, even if the box is soaked under the liquid sample or the box is exposed to a humid atmosphere for the analysis thereof.
The second surface plasmon sensor according to the present invention is different from the first surface plasmon sensor essentially only in that the second surface plasmon sensor is provided with the dielectric film layer in addition to the dielectric block, the light source, the optical system and the photodetection means. As the dielectric film layer is also housed in the box having the waterproof structure, the second surface plasmon sensor shows the same advantage as the first surface plasmon sensor.